1. Field of the Invention
The present invention relates to a device for determining the distance of a cutting tool, for instance a router bit, relative to a reference surface.
2. Description of the Related Art
A router is a tool, especially used in the woodworking industry, used for milling out (hollow out) an area in the face of a piece of material (workpiece). Usually the material is wood or metal, but applying the router to other types of materials is also possible. There are several types of routers. Nowadays, most common types are spindle type routers (spindle routers), where a router bit is mounted onto a rotatable spindle of an electric motor.
A typical router set-up includes a frame for supporting a router table. The workpiece is supported by the table. The table has an opening through which a cutting tool, in this case a router bit, protrudes for machining the material. Different bit sizes and shapes can be used, and the bit is easily exchangeable. The bit is engaged by a collet provided with a clamping mechanism such as a number of clamps or jaws. The collet is part of a rotatable spindle that is connected to a motor drive for rotation. The router collet (and router bit mounted in the mouth of the collet) can be rotated around an axis perpendicular to the router table. Sometimes the collet may be able to move with respect to the table, for example, along the axis of rotation by a depth adjustment tool. In this case, the shape of the cut that is created is determined by the size and shape of the bit (cutter) held in the collet and the height of the bit relative to the upper surface of the router table.
A typical router system also includes a router fence. The router fence is placed on top of the table and is used as a stop along which the workpiece can be guided during the machining thereof to ensure that the proper part of the workpiece is cut away. The router fence is used as a directional tool during the processing of the workpiece.
Three important variables determining the router's work are: 1) the shape of the router bit (the cutter); 2) the height of the router bit (in most cases the router machine is held in a router lift, that allows easy vertical travel); and 3) the distance between the router bit and a fence placed on top of the router table and used as a stop along which the work piece can be guided during the machining thereof. Regarding the first variable, a large variety of different router bit shapes are on the market nowadays, allowing an almost indefinite shape to be routed to the workpiece. In the second variable, variation of the router bit height, determines how much of the wood is actually shaped. The variation may be accomplished by a depth adjustment tool, for example a router lift. The router lift allows the entire router machine to be cranked up or down with great precision, exposing more or less of the router bit above the router table. However, the handling of the third variable remains burdensome and is currently to a large extent a process of trial and error.
The determine the second variable, that is the height of the router bit or, more specifically, the height of the actual projection of a specific portion of the router bit, for instance a cutting edge, relative to a reference surface, for instance the upper surface of a router table, a variety of measuring tools is available. One of the methods of measuring the height of the router bit is by direct measurement of the distance from the upper surface of the router table to the cutting edge of the router bit. A measuring device for measuring the projection of the cutting edge of a router bit and/or for setting the router bit height is disclosed in GB 2 386 085 B. The device comprises a bridging cursor body provided with a steel rule or scale extending perpendicular to the body and passing through a hole provided therein to a datum formed by a line drawn between two feet which form the bases of the device. In use, the feet of the tool are set vertically upon the router table, so that the cursor body bridges the opening in the router table and the router bit projecting through the opening. The rule or scale is moved downward to contact the upper end surface of the router bit. The height of the upper end surface of the router bit can then by determined from reading the indication on the rule or scale that represents the height of the scale relative to the router table.
Instead of or additionally to measuring the height of the projection of the router bit relative to the router table, i.e. actually obtaining a measurement value of the relevant distance, the determination of the height may be also involve the movement of the position of the router bit to set the height of its projection to a predetermined value. In this case the device is used to set the cutting tool relative to the reference surface.
However, the known device can only be used to determine the distance between the upper end surface of the router bit relative to an essentially horizontal reference plane formed by the table surface. Sometimes the reference for measuring the distance is a specific (side) portion of the router bit rather than the upper end surface thereof.
In another situation it is not the vertical projection of the router bit above the router table surface but rather the distance between a specific portion of the router bit tool and another reference surface, for instance a reference surface formed by the side of a router fence facing the router bit. The known device is not suited for measuring this distance. One may place the known device directly on the router table such that the feet of the tool are in contact with the router fence. However, it is only possible to measure the distance between the router fence and the router bit at table level. In practice, it is not the router bit at table level but rather the bit at a higher level that is to be used as reference for determining the distance to the router fence. Moreover, when using a lock miter bit on a router table, both the distance from the router fence and the distance from the table surface needs to be identical. This symmetry in distance is required at an exact spot on the cutting edge of the router bit which cutting edge is generally not at the table level.